usb: phy: replace spinlock with mutex in OTG FSM

OTG Final State Machine calls functions which may sleep.

For example, start_gadget callback implementation can use
usb_gadget_vbus_connect(), whose context: can sleep.

If so, mutex should be used instead of spinlock.

Signed-off-by: Anton Tikhomirov <av.tikhomirov@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>